The present invention relates to a thermosensitive recording material which is improved, in particular with respect to the stability of recorded images. The thermosensitive recording material comprises (a) a support member, (b) a thermosensitive coloring layer formed on the support member, comprising a leuco dye which is a colorless or light-colored at room temperature, and an antipyrine complex of zinc thiocyanate (hereinafter referred to as the zinc thiocyanate antipyrine complex) which serves as a color developer capable of inducing color formation in the leuco dye upon application of heat thereto, and (c) a protective layer formed on the thermosensitive coloring layer, consisting essentially of a water-soluble resin for protecting the thermosensitive coloring layer.
A conventional thermosensitive recording material comprises a support member such as a sheet of paper and a thermosensitive coloring layer formed on the support material, on which thermosensitive coloring layer colored images can be formed by application of heat thereto, for example, through thermal head, thermal pen, laser beams or strobo lamp.
Such thermosensitive recording materials are employed in a variety of fields, for instance, for use with printers of computers, recorders of medical analytical instruments, facsimile apparatus, automatic ticket vending apparatus, and thermosensitive copying apparatus, since they have the following advantages over other conventional recording materials. (1) Images can be formed by simple heat application, without complicated steps for development and image fixing, and therefore image recording can be speedily performed by a simple recording apparatus, without generation of noise and causing environmental pollution; and (2) the thermosensitive recording materials are inexpensive.
In such conventional thermosensitive recording materials, there is usually employed in the thermosensitive coloring layer a thermal coloring system comprising a combination of (i) a colorless or light-colored leuco dye containing a lactone ring, a lactam ring or a spiropyran ring, and (ii) a color developer which induces color formation in the leuco dye upon application of heat by the reaction with the leuco dye. As such color developers, for example, organic acids and phenolic materials are employed. Thermal coloring systems comprising a combination of such leuco dye and color developer are widely used, since the color tone of the images produced by such coloring systems is clear, the background of thermosensitive recording materials employing this coloring system is high in the degree of whiteness, and the produced images are highly resistant to light, so that the images do not easily fade.
As color developers for use with conventional leuco dyes, Bisphenol A and p-hydroxybenzoic acid esters are usually used. These phenolic compounds, however, have the shortcomings that produced images are so unstable that the image density gradually decreases with time even if the images are allowed to stand under normal office conditions, and in particular, when the images come into contact, for instance, with oils from hands, the image density is significantly decreased. Organic acids also have such shortcoming when they are used as color developers. Therefore, there is a strong demand for improvement of the stability of images developed in thermosensitive recording materials.
In the field of pressure sensitive recording sheet, metal compounds of aromatic carboxylic acids are proposed as being color developers capable of yielding stable images, as disclosed in Japanese Patent Publication No. 52-1327. When such metal compounds of aromatic carboxylic acids are applied for a conventional thermosensitive recording material, images are in fact stabilized. However, the non-image areas (background) of the thermosensitive recording material are easily discolored when coming into contact with oils, so that the developed images become illegible. Therefore, such metal compounds of aromatic carboxylic acids are not suitable for use in the thermosensitive recording materials.
In addition, as color developers, metal salts of aromatic carboxylic acids, such as metal salts of salicylic acid and 2-hydroxy-3-naphthoic acid, are also proposed as color developers. These color developers, when employed in thermosensitive recording materials, also have the shortcoming that developed images are significantly discolored when coming into contact with oils.
In order to eliminate the above conventional problems, the inventors of the present invention have proposed a thermosensitive recording material which is improved with respect to the stability of developed images by use of an antipyrine complex of zinc thiocyanate as the color developer for leuco dyes. This thermosensitive recording material is in fact improved with the stability of the developed images and background when coming into contact with oils, plasticizers and alcohols, as compared with conventional thermosensitive recording materials using Bisphenol A as the color developer for leuco dyes. However, it has the shortcomings that the image density of developed images is decreased when the images are brought into contact with a wrapping film made of polyvinyl chloride under application of pressure thereto at high humidities and that the background is colored or discolored when coming into contact with alcohols when the antipyrine complex is used in combination with particular leuco dyes.